Digital printing allows quick and easy modification of images to be printed. However, the speed of digital printing has generally lagged behind conventional printing processes such as lithographic printing and offset press processes. It is generally accepted that inks for digital printing differ in terms of composition and in terms of specification from inks for analogue printing and from formulations for paints industry. Typically, inks for analogue printing and formulations for paints show high viscosities (up to 3000-6000 cP), high particle size, high amount of colorants, fast drying. As a result of these properties, inks for analogue printing/formulations for paints cannot be pumped, cannot pass fine tubes (diameter 2-20 mm), and therefore clog nozzles of ink jet heads.
There are ongoing efforts to produce faster digital printer hardware. In particular, ink jet printers are improving in speed. Ink jet heads fire at higher frequencies to improve the speed of ink jet printers. The viscosity of jet inks has increased as higher frequency heads are used by ink jet printers.
Ink jet printers have traditionally used inks having a viscosity of below 10 centipoise (cP). For example, many Epson ink jet heads in common use inks having a viscosity of around 3-5 cP at 25° C. However, newer ink jet heads such as those from Aprion, Scitex Vision (Hewlett Packard), Spectra, Xaar, Ricoh, Kyocera and Seiko use inks having viscosity in the range of 7 to 15 cP at 25° C. Newer proposed printers use inks having even higher viscosity. For example, a Printos head uses inks having a range of viscosity between 20-100 cP at 25° C.
The use of higher viscosity inks presents several problems. Thickening agents and glycols are used to increase the viscosity of the ink. These higher viscosity inks comprise, for example, 40% to 45% (by weight) glycol. A higher percentage of glycol necessitates a relatively lower percentage of colorants, such as pigments or dyes, which results in a lower quality printed image, with reduced color vibrancy.
Further, the higher percentage of glycol or similar thickening agents and humectants results in an ink which does not easily dry on the printed surface such paper, plastic or textile when compared to more traditional aqueous inks having a higher percentage of water. The high percentage of glycols or similar agents may require the use of drying hoods or similar hardware to dry the ink after it is printed.
Yet another problem related to high viscosity inks is that the thicker inks require more energy to print the inks. The additional frequency of firing of the printer heads and the requirement of additional energy to force the more viscous ink through the printer heads results in a substantial amount of heat being generated at the print heads. Printers using higher viscosity inks may require a cooling apparatus, such as a circulating water system, that removes heat from the print heads. For example, the newer Printos head is able to fire high viscosity inks of 10-100 cP at 25° C., but a cooling system must be integrated to the printing heads to reduce the temperature of the heads and the inks. This requirement further increases the complexity of the head technology as well as the cost of printers, and the cost of use of the printers.
Thus, it is an object of the present invention is to mitigate at least some of these drawbacks of the state of the art. In particular, there is a need for an ink that will provide higher viscosity inks as needed for high speed ink jet printers. The desired ink should not have high levels materials that are difficult to dry, e.g. glycols. The desired ink should also allow high percentages of colorants to be used in the ink for improved print quality. The ink should reduce heat generation at the print head when compared to other inks of high viscosity.
These objectives are achieved by an ink jet ink as defined in claims 1 and 2. Further aspects of the invention are disclosed in the specification and independent claims, preferred embodiments are disclosed in the specification and the dependent claims.
The present invention will be described in more detail below. It is understood that the various embodiments, preferences and ranges as provided/disclosed in this specification may be combined at will. Further, depending of the specific embodiment, selected definitions, embodiments or ranges may not apply. It is also understood that the term “comprising” shall, in the context of this invention, also include the meaning of “containing” or “consisting of”.
The term “liquid” material denotes any material or component that is liquid under ambient conditions, such as water and glycol. The term “solid” material denotes any material or component that is solid under ambient conditions, such as pigments. Solid materials may be present “dispersed”, such as pigments, disperse dyes or resins, or “dissolved”, such as shear thinning agents. Dissolved materials may be present as a solution (ie. mono-phase), as a micellar solution or as an emulsion (i.e. bi-phase).
The term “print head” is known in the field and particularly denotes an ink jet head of an ink jet printer, such as commercially available from Epson, Aprion, Scitex Vision (Hewlett Packard), Spectra, Xaar, Ricoh, Seiko, Kyocera, Konica-Minolta, Trident, Imaje, Video-Jet, Canon, Memjet, Jemtex, Hitachi, Toshiba Tec.